In turbomachine systems without an auxiliary bearing, if a primary bearing (such as a magnetic bearing) fails, the shaft of the turbomachine will drop onto the adjacent mechanical components. This drop will cause substantial damage to the shaft and/or the surrounding components. In turbomachine systems that include an auxiliary bearing, also known as a “coast down bearing” or a “catcher bearing,” the auxiliary bearing will support the shaft in a stable position after failure of a primary bearing and allow for the safe coast down of the shaft.
In a typical arrangement, while a shaft is levitated by primary magnetic bearings during normal operation, the shaft does not contact the auxiliary bearing. When levitation by the primary magnetic bearings is lost, the shaft drops onto the auxiliary bearing. However, problems may arise during the operation of a typical auxiliary bearing arrangement. For example, the shaft may drop onto friction pads mounted on the auxiliary bearings. These friction pads may not stabilize the rotor adequately during coast down, especially if the shaft applies gravitational and rotational forces on a single pad (as opposed to between two or more pads). Moreover, when a drop of the shaft occurs, the auxiliary bearing is accelerated instantaneously to the shaft rotation speed. This configuration subjects the components of the auxiliary bearing to extreme accelerations, which have been shown to cause backward whirl of the shaft, brinelling of bearing races, skidding between bearing rolling elements and races, high stresses in the bearing housing, and overheating of the bearing. Therefore, the operating life of auxiliary bearings in these typical configurations is typically only a few drops of the shaft.
Further complicating the matter, magnetic bearings are now being considered for applications where the auxiliary bearings must repeatedly control deceleration of the shaft while minimizing damage to auxiliary bearing components. The lack of a reliable, long-lasting, auxiliary bearing technology has been a barrier to the implementation of magnetic bearings in certain turbomachines. Thus, there is a need for an auxiliary bearing system that can tolerate numerous drops of a shaft.